AIO tanks are expensive for what you are generally getting (you're paying for the compact and pretty factor with them). I've seen nano modders drop more money on their hoods and chillers than I would sink into an entire 90 gallon set-up. Sure, their tanks are cool, but there's no substitute for size once you've been collecting corals for a while (people either inevitably go larger or get out of the hobby... I've even seen people sell of their stuff to go smaller and end up back in and bigger then ever).
On the 55 idea... I see a lot of people moving up from freshwater and making a 55 gallon their first SW tank (heck, that's what we did). I'm sure somebody will disagree with me, but I think 55 gallon tanks are about as ill suited to a reef tank as you can get. With only 12" front-to-back it becomes very hard to get any rock in the tank without taking up all of your swimming room. Cool looking aquascaping... practically a pipe dream. Circulation also becomes something of a problem because of how long and skinny the tanks are. Getting a proper overflow in the tank also presents a problem because almost all 55's are made from tempered glass all the way around and because an overflow will take up valuable space in the tank (a coast-to-coast would literally take up almost half of the tank opening). If you can live with a siphon overflow, you can probably get away with it, but they aren't cheap and I couldn't sleep well at night having one on my tank. In short, I think I'd honestly rather have a smaller cube tank.
...but if you already have the tank and you'll only have to sink a little bit more money into it, it might be worth it.
In any case, if you do something in a 55 gallon range, my advice would be to buy everything you can used and buy slightly overrated equipment so that you can move up to a 75 gallon or 90 gallon without re-buying (they have a much more favorable footprint for reef tanks and it will make a future upgrade much cheaper).
As you probably know I have a 90 and a 92 gallon for sale. I realize that even if I drop quite a bit off of those tanks you'll still end up over $350 in order to get everything you need up and running on them (aside from any equipment, you'd need rock, which ain't cheap). Just wanted to put that out there. Economic stimulus check
